Thirteen million letters of genetic code from chromosome 21.
Groupings of letters of genetic code that are known to have a function
in the human body are called genes. The darker color depicts sequences
of code (called exons) that are known to be used by a cell as the set
of instructions for building a protein. These instructions are
interrupted by unused pieces of code (introns) which here have a
medium coloring. The gray areas have no currently known function.

Chromosome 21 is one of the shortest human chromosomes, yet this image
is only one quarter of the 50 million letters of which it is
composed. It would take 250 images of this size to depict the genetic
code in the entire human genome, which is 3.1 billion letters.